System and method for automatically verifying capture during multi-chamber stimulation

ABSTRACT

A system and corresponding method are provided to reliably detect capture during multi-chamber stimulation, and to further monitor the progression of congestive heart failure. The system provides a method by which intracardiac electrogram (IEGM) characteristics representing single-chamber capture and bi-ventricular capture are stored in memory and displayed. The annotation of the displayed waveforms is such that events associated with loss of capture, single-chamber capture, and bi-ventricular capture are clearly marked for ready interpretation by the physician. In a first situation, a stimulation pulse is followed by a time delay window and a subsequent depolarization complex that represents intrinsic responses of the chambers that have not been captured. In a second situation, a stimulation pulse is followed almost immediately by an evoked response that represents capture of one chamber, and a subsequent depolarization complex that represents an intrinsic response of one chamber that has not been captured. In a third situation, a stimulation pulse is almost immediately followed by an evoked response that represents simultaneous capture of two chambers.

PRIORITY CLAIM

[0001] This application claims the priority of copending provisionalU.S. application Ser. No. 60/203,688, filed May 11, 2000, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to a programmable cardiacstimulating apparatus for the purpose of automatically verifying captureduring multi-chamber stimulation. More specifically, the presentinvention is directed to an implantable stimulation device andassociated method for automatically verifying simultaneous captureduring bi-ventricular or bi-atrial stimulation, also referred to hereinas bi-chamber stimulation or two corresponding chamber stimulation.

BACKGROUND OF THE INVENTION

[0003] Congestive heart failure (CHF) is a debilitating, end-stagedisease in which abnormal function of the heart leads to inadequateblood flow to fulfill the needs of the body's tissues. Typically, theheart loses propulsive power because the cardiac muscle loses capacityto stretch and contract. Often, the ventricles do not adequately fillwith blood between heartbeats and the valves regulating blood flow maybecome leaky, allowing regurgitation or back flow of blood. Theimpairment of arterial circulation deprives vital organs of oxygen andnutrients. Fatigue, weakness, and inability to carry out daily tasks mayresult.

[0004] Not all CHF patients suffer debilitating symptoms immediately.Some may live actively for years. Yet, with few exceptions, the diseaseis relentlessly progressive.

[0005] As CHF progresses, it tends to become increasingly difficult tomanage. Even the compensatory responses it triggers in the body maythemselves eventually complicate the clinical prognosis. For example,when the heart attempts to compensate for reduced cardiac output, itadds muscle causing the ventricles to grow in volume in an attempt topump more blood with each heartbeat. This places a still higher demandon the heart's oxygen supply. If the oxygen supply falls short of thegrowing demand, as it often does, further injury to the heart mayresult. The additional muscle mass may also stiffen the heart walls tohamper rather than assist in providing cardiac output.

[0006] CHF has been classified by the New York Heart Association (NYHA).Their classification of CHF corresponds to four stages of progressivelyworsening symptoms and exercise capacity from Class I to Class IV. ClassI corresponds to no limitation wherein ordinary physical activity doesnot cause undue fatigue, shortness of breath, or palpitation. Class IIcorresponds to slight limitation of physical activity wherein suchpatients are comfortable at rest, but where ordinary physical activityresults in fatigue, shortness of breath, palpitations, or angina. ClassIII corresponds to a marked limitation of physical activity wherein,although patients are comfortable at rest, less than ordinary activitywill lead to symptoms. Class IV corresponds to inability to carry on anyphysical activity without discomfort, wherein symptoms of CHF arepresent even at rest and where with any physical activity, increaseddiscomfort is experienced.

[0007] Current standard treatment for heart failure is typicallycentered around medical treatment using ACE inhibitors, diuretics, anddigitalis. It has also been demonstrated that aerobic exercise mayimprove exercise tolerance, improve quality of life, and decreasesymptoms. Only an option in approximately 1 out of 200 cases, hearttransplantation is also available. Other cardiac surgery is alsoindicated for only a small percentage of patients with particularetiologies. Although advances in pharmacological therapy havesignificantly improved the survival rate and quality of life ofpatients, patients in NYHA Classes III or IV, who are still refractoryto drug therapy, have a poor prognosis and limited exercise tolerance.Cardiac pacing has been proposed as a new primary treatment for patientswith drug-refractory CHF.

[0008] By tracking the progression or regression of CHF more closely,treatments could be administered more effectively. Commonly, patientsadapt their lifestyle and activities to their physical condition. Theactivity level of the patients with NYHA Class III or IV would be muchlower than that of the patients with NYHA Class I or II. The change inlifestyle or activity level, due to the patient's heart condition, willbe reflected by activity and respiration physiological parameters.

[0009] Besides various assessments of the cardiac function itself,assessment of activity and respiration are typically performed. Thisincludes maximal exercise testing in which the heart rate and maximumventilation are measured during peak exertion. However, peak exerciseperformance has been found to not always correlate well withimprovements in a patient's clinical conditions. Therefore, sub-maximalexercise testing can also be performed, such as a six-minute walk test.While improvements in sub-maximal exercise may suggest an improvement inclinical condition, sub-maximal exercise performance can be variable inthat it is dependent on how the patient happens to be feeling on theparticular day of the test.

[0010] As CHF progresses, the dilation of the heart chambers alters thenormal conduction time of the electrical signals through the heart.These electrical signals coordinate the depolarization and subsequentcontraction of the heart chambers. Bi-ventricular pacing is expected toimprove the coordination of heart chambers by reducing the rightventricle (RV) contraction time and the left ventricle (LV) contractiontime, and by increasing the diastolic filling time.

[0011] One challenge in bi-ventricular pacing is the ability to detectand verify capture of both ventricles. Since the benefit ofbi-ventricular pacing is derived only when capture of both chambers isachieved, proper determination of pacing threshold for each ventricle,or both combined, is imperative to a successful therapy delivery. Duringdevice implantation, physicians often rely on ECG recordings to observewhen a stimulating pulse is of sufficient energy to cause heartcontraction, a condition known as “capture.” The lowest stimulationpulse energy sufficient to capture the heart is referred to as “capturethreshold.”

[0012]FIGS. 3A, 3B and 3C depict three surface ECG recordings for threeexemplary capture situations during bi-ventricular pacing. FIG. 3Arepresents a surface ECG recording during sub-threshold bi-ventricularpacing, and illustrates the failure to capture both the left and rightventricles. FIG. 3A shows a stimulation pulse 120 followed by a naturaldepolarization complex 124, with a time delay 125 therebetween. In FIG.3A neither ventricle is captured, and the intrinsic responses of bothventricles are represented by the depolarization complex 124.

[0013]FIG. 3B represents a surface ECG during bi-ventricular pacing inwhich the capture of only one ventricle (i.e., the right ventricle) butnot the other ventricle (i.e., the left ventricle) is achieved. Aventricular stimulation pulse 126 is followed immediately by adepolarization complex 127 which is a complex representing both theevoked response of the captured ventricle and the intrinsic response ofthe other ventricle that has not been captured. The evoked response tothe stimulation pulse 126 in one ventricle is conducted naturally to theother ventricle causing a second depolarization. The conducted responseof the other ventricle slightly lags the evoked response in the capturedventricle in accordance with the inter-ventricular conduction delay.This slight delay, however, is not distinguishable on the surface ECG.Since two distinct events are not easily discernible, recognition ofonly single-chamber capture versus bi-ventricular capture from the ECGrecording alone is quite difficult.

[0014]FIG. 3C represents a surface ECG during bi-ventricular pacing whensuccessful capture of both ventricles is achieved. A stimulation pulse128 is followed immediately by a depolarization complex 129 representingthe evoked response of both ventricles. This ECG recording appearsgenerally similar to the ECG recording of FIG. 3B in which only onechamber was captured. As a result, differentiation betweensingle-chamber capture (FIG. 3B) and bi-ventricular capture (FIG. 3C) istherefore difficult and impractical from a surface ECG recording. Aninappropriately selected ventricular stimulation pulse energy could beharmful to the patient if only one ventricle is captured because poorsynchronization between chambers could lead to arrhythmias.

[0015] Implantable cardiac stimulating devices contain sensing circuitryfor monitoring the patient's internal heartbeat signals. These internalheartbeat signals are commonly referred to as the intracardiacelectrogram (“IEGM”). Cardiac stimulating devices monitor the IEGM todetermine precisely when stimulation pulses should be applied. Forexample, some implantable cardiac stimulating devices such as demandpacemakers apply electrical stimulation pulses to the heart only in theevent that the patient's heart fails to beat properly on its own. Byapplying stimulation pulses only when needed, it is possible to avoidcompetition between the pulses applied by the device and the patient'sintrinsic cardiac rhythm.

[0016] Cardiac stimulating devices process the IEGM to determine whattype of electrical pulses should be applied to the patient's heart.Other cardiac devices, known as cardiac monitoring devices, are usedsolely to monitor the patient's cardiac condition. Cardiac monitoringdevices are similar to cardiac stimulating devices, but do not containpulse generating circuitry. Both cardiac stimulating devices and cardiacmonitoring devices process the IEGM to identify various cardiac events.For example, an implantable cardiac device with atrial sensing circuitrycan detect P-waves that accompany atrial contractions. Ventricularsensing circuitry can be used to detect R-waves that accompany thecontraction of the patient's ventricles.

[0017] Cardiac stimulating devices additionally process the IEGM inorder to verify that a stimulating pulse is of sufficient energy tocapture. The lowest capture threshold is sought in order to conservebattery energy while maintaining effective therapy delivery. Numerousschemes for processing the IEGM to determine threshold and to detectingcapture are described for example in U.S. Pat. No. 5,766,229 to Bornzin,U.S. Pat. No. 5,778,881 to Sun et al., and U.S. Pat. No. 5,324,310 toGreeninger et al.

[0018] However, conventional capture detection methods generally addressthe need to determine threshold and to verify capture in single-chamberpacing, specifically the right ventricle, or dual chamber pacing,specifically the right atrium and right or left ventricle. Therefore, aneed still exists to detect threshold and capture during multi-chamberpacing configurations, particularly during bi-ventricular pacing in CHFpatients.

[0019] In dual-chamber atrial-ventricular pacing, an atrial pulsegenerator and atrial sense amplifier are connected to the atrial lead,and a ventricular pulse generator and ventricular sense amplifier areconnected to a ventricular lead. This allows separate sensing of atrialevents and ventricular events to allow for distinct monitoring of atrialand ventricular threshold and capture detection. In a bi-ventricularpacing system, however, the ventricular channel can be bifurcated andconnected to both the right ventricle lead and the left ventricle lead,with typically only one ventricular sense amplifier and one ventricularpulse generator, thus preventing the ventricles from being monitored orpaced separately. Therefore, a method is needed that allows monitoringof the right ventricle and the left ventricle threshold and capturedetection using existing hardware or circuitry.

[0020] Furthermore, a method of tracking the progression or regressionof CHF during delivery of chronic pacing therapies would allow treatmentto be administered more effectively.

[0021] A number of attempts have been made previously to provide forchronic monitoring of physiological parameters associated with CHF usingimplantable cardiac devices, such as pacemakers, in conjunction withphysiological sensors. Reference is made to U.S. Pat. No. 5,518,001 toSnell et. al.; U.S. Pat. No. 5,944,745; U.S. Pat. No. 5,974,340 toKadhiresan; U.S. Pat. No. 5,935,081 to Kadhiresan; U.S. Pat. No.6,021,351 to Kadhiresan et al.

[0022] However, as CHF progresses, the dilation of the ventriclesincreases, causing inter-ventricular conduction time to increase.Therefore, it would be desirable to have a method that automatically andaccurately monitors inter-ventricular conduction time duringbi-ventricular pacing as a means for monitoring CHF progression.

SUMMARY OF THE INVENTION

[0023] The present invention addresses these needs by providing animplantable stimulation device that reliably detects bi-chamber captureduring multi-chamber pacing, measures the interchamber conduction delayto monitor the progression of CHF and to optimize therapy delivery.These goals are achieved without additional hardware or circuitry.

[0024] While the events of single-chamber capture verses both chambercapture are difficult to distinguish on a surface ECG, they are clearlyvisible on an IEGM.

[0025] The present invention is capable of sensing a composite cardiacsignal on a single sense channel that has, inherent in it,characteristics that permit the detection of non-capture, single-chambercapture, and bi-chamber capture. While the illustrated embodiments aredirected towards a bi-ventricular stimulation device, the presentinvention can be equally applied in a bi-atrial mode of stimulation.

[0026] Thus, one aspect of the present invention is to provide a methodby which an IEGM characteristic representing non-capture, single-chambercapture and bi-ventricular capture are stored in memory.

[0027] In one embodiment, the IEGM characteristics for non-capture,single-chamber capture and bi-ventricular capture are compared to newlyacquired IEGM waveforms during normal operation of the stimulationdevice, to determine whether the newly acquired IEGM represents failedcapture, single-chamber capture, or bi-ventricular capture.

[0028] In an alternative embodiment, sampled IEGM waveforms during bothsingle-chamber capture and bi-ventricular capture are processed by amorphology detector that measures and stores defining characteristics ofthe single-chamber capture and bi-ventricular capture. For example, peakdetection, slope detection, waveform integration, and timing intervalestimation can be performed with results stored in memory. Then, duringnormal operation of the stimulation device, a newly sampled IEGMwaveform can be processed in the same way to allow comparison of itswaveform characteristics to single-chamber capture or bi-ventricularcapture characteristics. In this way, reliable and automatic detectionof capture during bi-ventricular pacing is achieved.

[0029] A further aspect of the present invention is to provide atemporary high ventricular stimulation pulse energy if correlationbetween a newly sampled IEGM and the bi-ventricular template or thebi-ventricular waveform properties is poor. This high stimulation energyallows bi-ventricular capture to be regained. The device may then updatethe single-chamber capture and the bi-ventricular capture IEGMcharacteristics for future capture detection.

[0030] In one embodiment, the method of detecting bi-chamber capture mayexist in an external cardiac stimulation device, such as a temporarypacing device, a pacing system analyzer or a programmer capable ofperforming capture tests. In this embodiment, an automatic displayfeature allows for the display of the acquired IEGM waveforms. A furtherfeature is the annotation of the displayed waveforms such that eventsassociated with loss of capture, single-chamber capture, andbi-ventricular capture are clearly marked for the physician tointerpret.

[0031] Yet a further aspect of the present invention is to provide amethod by which progression of CHF can be monitored. During theacquisition phase of the single-chamber template or waveformcharacteristic, the time interval between the evoked response of thecaptured ventricle and the conducted response in the non-capturedventricle can be measured as an estimation of the inter-ventricularconduction time. This measurement can be stored in memory to beavailable during patient follow-up such that worsening of CHF can bedetected. Furthermore, a worsening or improving of the inter-ventricularconduction time could be used as feedback within the stimulation deviceto adjust stimulation parameters in a way that optimizes the stimulationtherapy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and further features, advantages and benefits of thepresent invention will be apparent upon consideration of the presentdescription taken in conjunction with the accompanying drawings, inwhich:

[0033]FIG. 1 is a simplified, partly cutaway view illustrating animplantable stimulation device in electrical communication with at leastthree leads implanted into a patient's heart for deliveringmulti-chamber stimulation and shock therapy;

[0034]FIG. 2 is a functional block diagram of the multi-chamberimplantable stimulation device of FIG. 1, illustrating the basicelements that provide cardioversion, defibrillation and/or pacingstimulation in four chambers of the heart;

[0035]FIG. 3A represents a conventional surface ECG recording duringsub-threshold bi-ventricular pacing, and illustrates the failure tocapture both the left and right ventricles;

[0036]FIG. 3B represents a conventional surface ECG duringbi-ventricular pacing in which only single-chamber capture is achieved;

[0037]FIG. 3C represents a conventional surface ECG duringbi-ventricular pacing in which successful capture of both ventricles isachieved;

[0038]FIG. 4A is a circuit diagram representing an equivalent circuit ofthe stimulation device of FIG. 1 for bi-ventricular pacing through abifurcated connector of the stimulation device of FIG. 1;

[0039]FIG. 4B is a circuit diagram representing an equivalent circuit ofthe stimulation device of FIG. 1 for bi-ventricular sensing;

[0040]FIG. 5A represents an IEGM recording during sub-thresholdbi-ventricular stimulation using the stimulation device of FIG. 1, andillustrates the failure to capture both the left and right ventricles;

[0041]FIG. 5B represents an IEGM during bi-ventricular stimulation usingthe stimulation device of FIG. 1, in which only single-chamber captureis achieved;

[0042]FIG. 5C illustrates an IEGM recording during bi-ventricularstimulation using the stimulation device of FIG. 1, in which successfulcapture of both ventricles is achieved;

[0043]FIG. 6 is a graph illustrating IEGM waveform characteristics thatcan be determined by a morphology detector in accordance with oneembodiment of the present invention;

[0044]FIG. 7 is a flow chart depicting a high level method used by thestimulation device of FIGS. 1 and 2 for verifying capture duringbi-ventricular (or bi-atrial) stimulation; and

[0045]FIG. 8 is a flow chart depicting a method used by the stimulationdevice of FIGS. 1 and 2 for determining the capture state, thecorresponding IEGM waveform characteristics during bi-ventricular (orbi-atrial) stimulation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] The following description is of a best mode presentlycontemplated for practicing the invention. This description is not to betaken in a limiting sense but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be ascertained with reference to the issued claims. Inthe description of the invention that follows, like numerals orreference designators will be used to refer to like parts or elementsthroughout. While the following description is directed towards abi-ventricular method of stimulating the heart, the present inventionincludes applying the method in both of the atria to perform bi-atrialstimulation.

[0047]FIG. 1 illustrates a stimulation device 10 in electricalcommunication with a patient's heart 12 by way of three leads 20, 24 and30 suitable for delivering multi-chamber stimulation and shock therapy.To sense atrial cardiac signals and to provide right atrial chamberstimulation therapy, the stimulation device 10 is coupled to animplantable right atrial lead 20 having at least an atrial tip electrode22, which typically is implanted in the patient's right atrialappendage.

[0048] To sense left atrial and ventricular cardiac signals and toprovide left-chamber stimulation therapy, the stimulation device 10 iscoupled to a “coronary sinus” lead 24 designed for placement in the“coronary sinus region” via the coronary sinus os so as to place adistal electrode adjacent to the left ventricle and additionalelectrode(s) adjacent to the left atrium. As used herein, the phrase“coronary sinus region” refers to the vasculature of the left ventricle,including any portion of the coronary sinus, great cardiac vein, leftmarginal vein, left posterior ventricular vein, middle cardiac vein,and/or small cardiac vein or any other cardiac vein accessible by thecoronary sinus.

[0049] Accordingly, the coronary sinus lead 24 is designed to receiveatrial and ventricular cardiac signals and to deliver: left ventricularstimulation therapy using at least a left ventricular tip electrode 26,left atrial stimulation therapy using at least a left atrial ringelectrode 27, and shocking therapy using at least a left atrial coilelectrode 28. For a more detailed description of a coronary sinus lead,reference is made to U.S. Pat. application No. 09/457,277, filed Dec. 8,1999, titled “A Self-Anchoring, Steerable Coronary Sinus Lead” (Piancaet. al), which is a continuation-in-part of application Ser. No.09/196,898, filed Nov. 20, 1998 (now abandoned), which is incorporatedherein by reference.

[0050] The stimulation device 10 is also shown in electricalcommunication with the patient's heart 12 by way of an implantable rightventricular lead 30 having, in this embodiment, a right ventricular tipelectrode 32, a right ventricular ring electrode 34, a right ventricular(RV) coil electrode 36, and an SVC coil electrode 38. Typically, theright ventricular lead 30 is transvenously inserted into the heart 12 soas to place the right ventricular tip electrode 32 in the rightventricular apex so that the RV coil electrode 36 will be positioned inthe right ventricle and the SVC coil electrode 38 will be positioned inthe superior vena cava. Accordingly, the right ventricular lead 30 iscapable of receiving cardiac signals, and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

[0051]FIG. 2 illustrates a simplified block diagram of the multi-chamberimplantable stimulation device 10, which is capable of treating bothfast and slow arrhythmias with stimulation therapy, includingcardioversion, defibrillation, and stimulation stimulation. While aparticular multi-chamber device is shown, this is for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate, or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and/or pacing stimulation.

[0052] The stimulation device 10 includes a housing 40 which is oftenreferred to as “can”, “case” or “case electrode”, and which may beprogrammably selected to act as the return electrode for all “unipolar”modes. The housing 40 may further be used as a return electrode alone orin combination with one or more of the coil electrodes 28, 36, or 38,for shocking purposes. The housing 40 further includes a connector (notshown) having a plurality of terminals, 42, 44, 46, 48, 52, 54, 56, and58 (shown schematically and, for convenience, the names of theelectrodes to which they are connected are shown next to the terminals).As such, to achieve right atrial sensing and stimulation, the connectorincludes at least a right atrial tip terminal 42 adapted for connectionto the atrial (A_(R)) tip electrode 22.

[0053] To achieve left chamber sensing, pacing and/or shocking, theconnector includes at least a left ventricular (V_(L)) tip terminal 44,a left atrial (A_(L)) ring terminal 46, and a left atrial (A_(L))shocking terminal (coil) 48, which are adapted for connection to theleft ventricular tip electrode 26, the left atrial tip electrode 27, andthe left atrial coil electrode 28, respectively.

[0054] To support right chamber sensing, pacing and/or shocking, theconnector further includes a right ventricular (V_(R)) tip terminal 52,a right ventricular (V_(R)) ring terminal 54, a right ventricular (RV)shocking terminal (coil) 56, and an SVC shocking terminal (coil) 58,which are adapted for connection to the right ventricular tip electrode32, right ventricular ring electrode 34, the RV coil electrode 36, andthe SVC coil electrode 38, respectively.

[0055] At the core of the stimulation device 10 is a programmablemicrocontroller 60 that controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 typicallyincludes a microprocessor, or equivalent control circuitry, designedspecifically for controlling the delivery of stimulation therapy, andmay further include RAM or ROM memory, logic and timing circuitry, statemachine circuitry, and I/O circuitry. Typically, the microcontroller 60includes the ability to process or monitor input signals (data) ascontrolled by a program code stored in a designated block of memory. Thedetails of the design and operation of the microcontroller 60 are notcritical to the present invention. Rather, any suitable microcontroller60 may be used that carries out the functions described herein. The useof microprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

[0056] As shown in FIG. 2, an atrial pulse generator 70 and aventricular pulse generator 72 generate pacing stimulation pulses fordelivery by the right atrial lead 20, the right ventricular lead 30,and/or the coronary sinus lead 24 via a switch bank 74. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart, the atrial pulse generator 70 and the ventricularpulse generator 72 may include dedicated, independent pulse generators,multiplexed pulse generators, or shared pulse generators. The atrialpulse generator 70 and the ventricular pulse generator 72 are controlledby the microcontroller 60 via appropriate control signals 76 and 78,respectively, to trigger or inhibit the stimulation pulses.

[0057] The microcontroller 60 further includes timing control circuitry79 which is used to control the timing of such stimulation pulses (e.g.pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A)delay, or ventricular interconduction (V-V) delay, etc.), as well as tokeep track of the timing of refractory periods, PVARP intervals, noisedetection windows, evoked response windows, alert intervals, markerchannel timing, etc.

[0058] The switch bank 74 includes a plurality of switches forconnecting the desired electrodes to the appropriate I/O circuits,thereby providing complete electrode programmability. Accordingly, theswitch bank 74, in response to a control signal 80 from themicrocontroller 60, determines the polarity of the stimulation pulses(e.g. unipolar, bipolar, combipolar, etc.) by selectively closing theappropriate combination of switches (not shown) as is known in the art.

[0059] Atrial sensing circuits 82 and ventricular sensing circuits 84may also be selectively coupled to the right atrial lead 20, coronarysinus lead 24, and the right ventricular lead 30, through the switchbank 74, for detecting the presence of cardiac activity in each of thefour chambers of the heart. Accordingly, the atrial and ventricularsensing circuits 82 and 84 may include dedicated sense amplifiers,multiplexed amplifiers, or shared amplifiers. The switch bank 74determines the “sensing polarity”of the cardiac signal by selectivelyclosing the appropriate switches. In this way, the clinician may programthe sensing polarity independent of the stimulation polarity.

[0060] Each of the atrial sensing circuit 82 or the ventricular sensingcircuit 84 preferably employs one or more low power, precisionamplifiers with programmable gain and/or automatic gain control,bandpass filtering, and a threshold detection circuit, to selectivelysense the cardiac signal of interest. The automatic gain control enablesthe stimulation device 10 to deal effectively with the difficult problemof sensing the low amplitude signal characteristics of atrial orventricular fibrillation.

[0061] The outputs of the atrial and ventricular sensing circuits 82 and84 are connected to the microcontroller 60 for triggering or inhibitingthe atrial and ventricular pulse generators 70 and 72, respectively, ina demand fashion, in response to the absence or presence of cardiacactivity, respectively, in the appropriate chambers of the heart. Theatrial and ventricular sensing circuits 82 and 84, in turn, receivecontrol signals over signal lines 86 and 88 from the microcontroller 60,for controlling the gain, threshold, polarization charge removalcircuitry (not shown), and the timing of any blocking circuitry (notshown) coupled to the inputs of the atrial and ventricular sensingcircuits 82 and 84.

[0062] For arrhythmia detection, the stimulation device 10 utilizes theatrial and ventricular sensing circuits 82 and 84 to sense cardiacsignals, for determining whether a rhythm is physiologic or pathologic.As used herein “sensing” is reserved for the noting of an electricalsignal, and “detection” is the processing of these sensed signals andnoting the presence of an arrhythmia. The timing intervals betweensensed events (e.g. P-waves, R-waves, and depolarization signalsassociated with fibrillation which are sometimes referred to as“F-waves” or “Fib-waves”) are then classified by the microcontroller 60by comparing them to a predefined rate zone limit (e.g. bradycardia,normal, low rate VT, high rate VT, and fibrillation rate zones) andvarious other characteristics (e.g. sudden onset, stability, physiologicsensors, and morphology, etc.) in order to determine the type ofremedial therapy that is needed (e.g. bradycardia pacing,anti-tachycardia pacing, cardioversion shocks or defibrillation shocks,collectively referred to as “tiered therapy”).

[0063] Cardiac signals are also applied to the inputs of ananalog-to-digital (A/D) data acquisition system 90. The data acquisitionsystem 90 is configured to acquire intracardiac electrogram signals,convert the raw analog data into digital signals, and store the digitalsignals for later processing and/or telemetric transmission to anexternal device 102. The data acquisition system 90 is coupled to theright atrial lead 20, the coronary sinus lead 24, and the rightventricular lead 30 through the switch bank 74 to sample cardiac signalsacross any pair of desired electrodes.

[0064] Advantageously, the data acquisition system 90 may be coupled tothe microcontroller 60 or another detection circuitry, for detecting anevoked response from the heart 12 in response to an applied stimulus,thereby aiding in the detection of “capture”. Capture occurs when anelectrical stimulus applied to the heart is of sufficient energy todepolarize the cardiac tissue, thereby causing the heart muscle tocontract. The microcontroller 60 detects a depolarization signal duringa window following a stimulation pulse, the presence of which indicatesthat capture has occurred. The microcontroller 60 enables capturedetection by triggering the ventricular pulse generator 72 to generate astimulation pulse, starting a capture detection window using the timingcircuitry within the microcontroller 60, and enabling the dataacquisition system 90 via control signal 92 to sample the cardiac signalthat falls in the capture detection window and, based on the amplitudeof the sampled cardiac signal, determines if capture has occurred.

[0065] The microcontroller 60 is further coupled to a memory 94 by asuitable data/address bus 96, wherein the programmable operatingparameters used by the microcontroller 60 are stored and modified, asrequired, in order to customize the operation of the stimulation device10 to suit the needs of a particular patient. Such operating parametersdefine, for example, stimulation pulse amplitude, pulse duration,electrode polarity, rate, sensitivity, automatic features, arrhythmiadetection criteria, and the amplitude, waveshape and vector of eachshocking pulse to be delivered to the patient's heart 12 within eachrespective tier of therapy. A feature of the stimulation device 10 isthe ability to sense and store a relatively large amount of data (e.g.from the data acquisition system 90), which data may then be used forsubsequent analysis to guide the programming of the stimulation device10.

[0066] The operating parameters of the stimulation device 10 may benon-invasively programmed into the memory 94 through a telemetry circuit100 in telemetric communication with the external device 102, such as aprogrammer, transtelephonic transceiver, or a diagnostic systemanalyzer. The telemetry circuit 100 is activated by the microcontroller60 by a control signal 106. The telemetry circuit 100 advantageouslyallows intracardiac electrograms and status information relating to theoperation of the stimulation device 10 (as contained in themicrocontroller 60 or memory 94) to be sent to the external device 102through the established communication link 104.

[0067] In a preferred embodiment, the stimulation device 10 furtherincludes a physiologic sensor 108, commonly referred to as a“rate-responsive” sensor because it is typically used to adjust pacingstimulation rate according to the exercise state of the patient.However, the physiological sensor 108 may further be used to detectchanges in cardiac output, changes in the physiological condition of theheart, or diurnal changes in activity (e.g. detecting sleep and wakestates). Accordingly, the microcontroller 60 responds by adjusting thevarious pacing parameters (such as rate, AV Delay, V-V Delay, etc.) atwhich the atrial and ventricular pulse generators 70 and 72 generatestimulation pulses.

[0068] While the physiologic sensor 108 is shown as being includedwithin the stimulation device 10, it is to be understood that thephysiologic sensor 108 may alternatively be external to the stimulationdevice 10, yet still be implanted within, or carried by the patient. Acommon type of rate responsive sensor is an activity sensor, such as anaccelerometer or a piezoelectric crystal, which is mounted within thehousing 40 of the stimulation device 10. Other types of physiologicsensors are also known, for example, sensors that sense the oxygencontent of blood, pressure, cardiac output, ejection fraction, strokevolume, end diastolic volume, end systolic volume, respiration rateand/or minute ventilation, pH of blood, ventricular gradient, etc.However, any sensor may be used which is capable of sensing aphysiological parameter that corresponds to the exercise state of thepatient.

[0069] The stimulation device 10 additionally includes a power sourcesuch as a battery 110 that provides operating power to all the circuitsshown in FIG. 2. For the stimulation device 10, which employs shockingtherapy, the battery 110 must be capable of operating at low currentdrains for long periods of time, and also be capable of providinghigh-current pulses (for capacitor charging) when the patient requires ashock pulse. The battery 110 must preferably have a predictabledischarge characteristic so that elective replacement time can bedetected. Accordingly, the stimulation device 10 can employlithium/silver vanadium oxide batteries.

[0070] The stimulation device 10 further includes a magnet detectioncircuitry (not shown), coupled to the microcontroller 60. The purpose ofthe magnet detection circuitry is to detect when a magnet is placed overthe stimulation device 10, which magnet may be used by a clinician toperform various test functions of the stimulation device 10 and/or tosignal the microcontroller 60 that an external programmer 102 is inplace to receive or transmit data to the microcontroller 60 through thetelemetry circuit 100.

[0071] As further shown in FIG. 2, the stimulation device 10 is shown ashaving an impedance measuring circuit 112 which is enabled by themicrocontroller 60 by a control signal 114. Certain applications for animpedance measuring circuit 112 include, but are not limited to, leadimpedance surveillance during the acute and chronic phases for properlead positioning or dislodgment; detecting operable electrodes andautomatically switching to an operable pair if dislodgment occurs;measuring respiration or minute ventilation; measuring thoracicimpedance for determining shock thresholds; detecting when the devicehas been implanted; measuring stroke volume; and detecting the openingof the valves, etc. The impedance measuring circuit 112 isadvantageously coupled to the switch bank 74 so that any desiredelectrode may be used.

[0072] It is a function of the stimulation device 10 to operate as animplantable cardioverter/defibrillator (ICD) device. That is, it mustdetect the occurrence of an arrhythmia, and automatically apply anappropriate electrical shock therapy to the heart aimed at terminatingthe detected arrhythmia. To this end, the microcontroller 60 furthercontrols a shocking circuit 116 by way of a control signal 118. Theshocking circuit 116 generates shocking pulses of low (up to 0.5Joules), moderate (0.5-10 Joules), or high (11 to 40 Joules) energy, ascontrolled by the microcontroller 60. Such shocking pulses are appliedto the patient's heart through at least two shocking electrodes, and asshown in this embodiment, selected from the left atrial coil electrode28, the RV coil electrode 36, and/or the SVC coil electrode 38 (FIG. 1).As noted above, the housing 40 may act as an active electrode incombination with the RV electrode 36, or as part of a split electricalvector using the SVC coil electrode 38 or the left atrial coil electrode28 (i.e., using the RV electrode as common electrode).

[0073] Cardioversion shocks are generally considered to be of low tomoderate energy level (so as to minimize pain felt by the patient),and/or synchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40Joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

[0074]FIG. 7 illustrates a flow chart describing an overview of theoperation and features implemented in one embodiment of the stimulationdevice 10. In this flow chart, and the other flow charts describedherein, the various algorithmic steps are summarized in individual“blocks”. Such blocks describe specific actions or decisions that mustbe made or carried out as the algorithm proceeds. Where themicrocontroller 60 (or its equivalent) is employed, the flow chartspresented herein provide the basis for a “control program” that may beexecuted or used by such a microcontroller 60 (or its equivalent) toeffectuate the desired control of the stimulation device.

[0075] With reference to FIG. 2, and in accordance with the presentinvention, a morphology detector 64 is incorporated in themicrocontroller 60 to allow for the processing of the sensedintra-cardiac electrogram signals (IEGM). IEGM sensing is achieved byreceiving the atrial IEGM signals along the right atrial lead 20 throughthe atrial sensing circuit 82, or by receiving the ventricular IEGMsignals along the right ventricular lead 30 or coronary sinus lead 24through the ventricular sensing circuit 84.

[0076] In the embodiment described herein, the control program iscomprised of multiple integrated program modules, with each modulebearing responsibility for controlling one or more functions of thestimulation device 10. For example, one program module may control thedelivery of stimulating pulses to the heart 12, while another maycontrol the verification of ventricular capture and ventricular pacingenergy determination. In effect, each program module is a controlprogram dedicated to a specific function or set of functions of thestimulation device 10. In particular, a program module is implemented bythe stimulation device 10 to perform capture verification duringmulti-chamber stimulation, more specifically during bi-ventricularstimulation.

[0077] In the stimulation device 10 or an equivalent system,bi-ventricular stimulation is achieved through the delivery of aventricular stimulation pulse from the ventricular pulse generator 72through the right ventricular lead 30 to stimulate the right ventricle,and the coronary sinus lead 24 to stimulate the left ventricle. Often,the output of the ventricular pulse generator 72 is connected to theright ventricular lead 30 and the coronary sinus lead 24 through abifurcated connector.

[0078] The equivalent circuit 130 for bi-ventricular stimulation througha bifurcated connector is shown in FIG. 4A, where the total current, I,delivered to both ventricles is represented as the sum of the current,I_(R), delivered to the right ventricle (RV), and the current, I_(L),delivered to the left ventricle (LV). The current delivered to eachventricle will be equal the voltage, V, produced by the ventricularpulse generator 72, divided by the respective impedance of eachventricle as given by the following equations that correspond to theright ventricle and left ventricle, respectively:

[0079] I_(R)=V/Z_(R), and

[0080] I_(L)=V/Z_(L),

[0081] where Z_(R) represents the total impedance through the rightventricle, and Z_(L) represents the total impedance through the leftventricle.

[0082] For example, when the impedance Z_(L) is greater than theimpedance Z_(R), then the current I_(L) will be less than the currentI_(R). Thus, during bi-ventricular pacing, the right ventricle istypically captured at a lower stimulation pulse amplitude than the leftventricle. This leads to three capture situations during bi-ventricularstimulation: The first situation being no capture in either the rightventricle or the left ventricle due to sub-threshold stimulation; thesecond situation being capture in one ventricle only, typically theright ventricle; and the third situation being capture in both the leftand the right ventricles. Therefore, it is desirable to distinguishthese three capture situations based on the sensed IEGM. Since,according to one embodiment, the ventricular sensing circuit 84 isconnected to both the right ventricular lead 30 and the coronary sinuslead 24 through the bifurcated connector, only one analog signal isreceived by the ventricular sensing circuit 84 providing input data fromboth the right ventricle and the right ventricle.

[0083] The equivalent circuit 132 for the situation of bi-ventricularsensing is generally depicted in FIG. 4B. The evoked response, ER,measured by the ventricular sensing circuit 84, reflects thecontribution of the evoked response ER_(R) in the right ventricle, andthe evoked response, ER_(L), in the left ventricle. The parallel load ofthe sensing impedances Z_(R) and Z_(L) represents the connection to theright ventricle and the left ventricle, respectively, from theventricular sensing circuit 84 that possesses an impedance Z_(A). Z_(R)represents the total sensing impedance through the right ventricle, andZ_(L) represents the total sensing impedance through the left ventricle.

[0084] For example, when the impedance Z_(L) is greater than theimpedance Z_(R), then the evoked response in the right ventricle,ER_(R), presents a greater contribution than the evoked response in theleft ventricle, ER_(L), in the combined evoked response, ER, as measuredby the ventricular sensing circuit.

[0085] In the situation where only one ventricle is captured, aconducted response in the other (or non-captured) ventricle will bedelayed in time following the evoked response, ER, signal produced bythe captured ventricle. Thus, two events will be detected by theventricular sensing circuit 84.

[0086] As it will be appreciated from the description of FIGS. 5Athrough 5C, the morphology of the IEGM waveform is distinctly differentduring the above three capture situations. FIGS. 5A through 5Cillustrate a method, according to one embodiment of the presentinvention, that enables the reliable detection of capture duringbi-ventricular stimulation based on IEGM morphology.

[0087]FIG. 5A represents an IEGM recording during sub-thresholdbi-ventricular stimulation, and illustrates the failure to capture boththe left and right ventricles. A stimulation pulse 134 is followed by atime delay window 136, and a subsequent depolarization complex 138 (e.g.an intrinsic R-wave) that represents the intrinsic responses of theright and left ventricles. In this situation, the stimulation pulseamplitude is too low to depolarize either ventricle, and the naturaldepolarization is represented by the complex 138 associated with thedepolarization of both ventricles.

[0088]FIG. 5B represents an IEGM during bi-ventricular stimulation inwhich only single-chamber capture is achieved. In this situation, thestimulation pulse 144 is followed immediately by an evoked response 146that represents capture of one ventricle (i.e., the right ventricle),and a subsequent intrinsic depolarization complex 148 that correspondsto the conducted response in the ventricle that has not been captured(i.e., the left ventricle).

[0089] This recording illustrates how the IEGM can be used to clearlydiscern between single-chamber and bi-ventricular capture. Thestimulation pulse amplitude is sufficient to capture one ventricle(typically the right ventricle) as evidenced by the evoked response 146,but not sufficiently enough to capture the other ventricle (typicallythe left ventricle) as evidenced by the latent depolarization complex148. The two responses 146 and 148 are distinct on the IEGM recording,and significantly simpler to discern compared to the two events thatappear as a single complex 127 on the surface ECG recording of FIG. 3B.

[0090]FIG. 5C illustrates an IEGM recording during bi-ventricularstimulation in which successful capture of both ventricles is achieved.In this situation, a stimulation pulse 150 is immediately followed by anevoked response 152. No latent conducted response occurs, verifying thatan evoked response occurred simultaneously in both ventricles with bothresponses represented by the evoked response 152.

[0091] In one embodiment of the present invention, the microprocessor 60(FIG. 2) processes the IEGM waveforms and detects a number of parametersor characteristics defining the IEGM morphology. Exemplarycharacteristics are illustrated in FIG. 6 and include but are notlimited to:

[0092] 1) template representation of the overall IEGM waveform 160;

[0093] 2) peak negative amplitude 162;

[0094] 3) peak positive amplitude 163;

[0095] 4) positive slope 164;

[0096] 5) negative slope 165;

[0097] 6) positive integral 166;

[0098] 7) negative integral 167;

[0099] 8) number of inflection points or zero crossings 168, 169, 170;

[0100] 9) time duration (width) of depolarizations 172;

[0101] 10) time interval 177 between the ventricular stimulation pulseand any subsequently detected events; and/or

[0102] 11) time interval between detected events (not shown).

[0103] One or more of these IEGM characteristics are then used by themethod of the present invention, as it will be described below, in orderto distinguish between single-chamber capture, bi-ventricular capture,or complete loss of capture, based on comparisons made between anacquired IEGM during normal stimulation device operation and the knowncharacteristics of the IEGM during the three capture situations.

[0104] In a preferred embodiment, IEGM characteristics representing thetypical morphologies of the IEGM during (1) single-chamber capture, and(2) bi-ventricular capture, are stored in memory 94 (FIG. 2). These IEGMcharacteristics are acquired and stored during threshold testingperformed at the time of device implant or at a follow-up office visit.During threshold testing, the ventricular stimulation pulse amplitude isprogressively increased in small steps until single-chamber capture isrecognized on the IEGM display. Once single-chamber capture is verified,the IEGM waveform is stored in memory as the single-chamber capturetemplate.

[0105] Next, the ventricular stimulation pulse amplitude is furtherincreased until bi-ventricular capture is recognized on the IEGMdisplay. The IEGM waveform associated with bi-ventricular capture isthen stored in memory as the bi-ventricular capture template. Otherthreshold-searching algorithms that are available to those practiced inthe art, such as progressively decreasing the ventricular stimulationpulse amplitude, can also be used successfully in the implementation ofthe present invention for obtaining and storing single-chamber andbi-ventricular capture IEGM characteristics.

[0106] Thus, one important feature of the present invention is anautomatic display feature which acquires and displays IEGM waveformmorphologies. The automatic display may be annotated such that IEGMevents, e.g. single-chamber evoked responses, bi-ventricular evokedresponses, conducted responses, and bi-ventricular intrinsic responsesare clearly indicated. Such annotation allows a medical practitioner toeasily distinguish between the various capture situations.

[0107]FIG. 7 is a flow-chart that illustrates a high level method forautomatically verifying stimulation capture in one or more cardiacchambers according to one embodiment of the present invention. Themethod starts at step 192 by generating stimulation pulses.

[0108] At step 193, the method detects intracardiac electrogram (IEGM)signals in the one or more chambers subsequent to the delivery of thestimulation pulses. Using the IEGM signals detected at step 193, themethod 191 distinguishes among three capture situations in the cardiacchambers, at step 194. These three capture situations include: a firstsituation that depicts the absence of capture in both chambers due, forexample, to sub-threshold stimulation; a second situation that depictscapture in only one of the cardiac chambers; and a third situation thatdepicts capture in two chambers.

[0109] In the event that there is non-capture or single-chamber capture(steps 195 or 196, respectively), then the loss of capture counter isincremented (step 200), and the stimulation energy is increased in step202. The method then returns to step 206 and continues with the pacingroutine for the next pacing cycle.

[0110] In the event that there is capture in both chambers (step 197),then the loss of capture counter is reset to zero. The method then alsoreturns to step 206 and continues with the pacing routine for the nextpacing cycle. The purpose of the loss of capture counter is to detect“n” loss of capture beats such that a threshold search can be triggered.

[0111] With reference to FIG. 8, a threshold search method could beinitiated at step 305 by the microcontroller 60 when loss ofbi-ventricular capture is suspected (e.g., after “n” loss of captureevents) or after every “m” hours as programmed or as needed at implantor at follow-up. The method is initiated at step 305 by increasing theventricular stimulation pulse amplitude, P_(V), to a maximal value,P_(VMAX). The IEGM waveform is then sampled and stored at step 310. TheIEGM waveform is processed by the morphology detector 64 such thatmicroprocessor 60 can verify that bi-ventricular capture has indeedoccurred.

[0112] Step 315 represents an algorithm in which IEGM properties asdetermined by the morphology detector 64 are compared to specificcriteria that would indicate bi-ventricular capture. A plurality ofparameters may be used for determining if a single depolarizationcomplex, representing the evoked response of both ventricles, hasoccurred immediately following the stimulation pulse. For example, withreference to FIG. 6, the number of negative peaks 162 detected within asampling window 190 would indicate if one or more depolarizations haveoccurred following the stimulation pulse. Timing intervals may also becompared. For example, if a negative peak 162 is detected within a shortperiod of time, for example 16 to 40 msec, the depolarization isinterpreted as an evoked response. If the depolarization occurs later intime but still within the sampling window 190, the depolarization isinterpreted as an intrinsic response indicating failure to captureeither ventricle, even at the temporary high stimulation energy. Thissituation would imply a system failure that would be flagged in memoryat the termination step 317.

[0113] Once bi-ventricular capture is verified at step 315, the IEGMwaveform is stored as the new bi-ventricular template at step 320 andfurther processed by the morphology detector 64 such that thebi-ventricular waveform characteristics can be stored in memory 94 (FIG.2).

[0114] Next, the ventricular stimulation pulse amplitude, P_(V), isdecreased at step 325 by a pre-defined value, p. Another IEGM waveformis sampled and stored at step 330. At decision step 335, the new IEGMwaveform is examined in a similar manner as in step 315 but this timethe algorithm tests for criteria indicating single-chamber capture, thatis two distinct depolarizations occurring after the stimulation pulse(FIG. 5B). For example, two depolarizations may be detected by twonegative peaks (FIG. 6) following the stimulation pulse within thesampling window 190, a second event detection at some time intervalafter a first event detection, a large value of the negative integral,or any of a number of other methods associated with the characteristicsdetermined by the morphology detector 64 and illustrated, for example,in FIG. 6.

[0115] If the criteria required to verify single-chamber capture are notmet, the ventricular stimulation pulse amplitude, P_(V), is decreasedagain at step 325, and the foregoing process is repeated untilsingle-chamber capture is verified. Once detection of single-chambercapture is verified, the IEGM waveform is stored in memory as the newsingle-chamber capture template at step 340 and further processed by themorphology detector 64 such that the single-chamber capture waveformcharacteristics can be stored in memory 94 (FIG. 2). Furthermore, thetime delay between two negative peaks, the inter-chamber conductiondelay, can be measured and stored at step 340.

[0116] The lowest setting at which bi-ventricular capture continues tooccur, that is the pulse energy prior to the last decrement of theventricular stimulation pulse amplitude, P_(V), is stored in the memory94 at step 345 as the bi-ventricular stimulation threshold, T. At step347, the ventricular pulse amplitude P_(V) is set equal to T, or T plussome programmed safety margin (SM), and the method 300 is terminated atblock 350. In this way, bi-ventricular capture is regained with thenewly acquired capture verification IEGM characteristics stored inmemory 94, and the stimulation device 10 can return to normal operationwith ongoing monitoring of bi-ventricular capture.

[0117] A further aspect of the present invention is the ability tomonitor the progression of CHF. This is achieved through thedetermination of temporal characteristics of the IEGM waveform,particularly during single-chamber capture by the morphology detector64. As the severity of CHF worsens, inter-ventricular conduction timeincreases due to further dilation of the ventricles. IEGM acquisitionand storage therefore provides a method for monitoring inter-ventricularconduction time as a means for monitoring the progression of CHF. If,during single-chamber capture, the time elapsed between a negative peakdetection 146 (FIG. 5B) to the next negative peak detection 148increases, which indicates that interventricular conduction delay hasworsened such that hemodynamic performance has been deteriorating.

[0118] An added feature of the present invention, as aforementioned inStep 340 in FIG. 8, is a means to monitor CHF progression by monitoringinter-ventricular conduction time during bi-ventricular thresholdtesting and IEGM template acquisition. Inter-ventricular conductiontime, measured as the time between two negative deflectingdepolarizations 146, 148, is stored in memory 94 (FIG. 2), and is madeavailable to the physician during patient follow-up visits.Inter-ventricular conduction time could also be used as a closed-loopfeedback parameter within the stimulation device 10 to cause automaticadjustments of pacing parameters such that pacing therapy is optimizedas CHF worsens or improves.

[0119] Thus, an implantable cardiac device and method for reliablydetecting and verifying capture during bi-ventricular pacing as well asa means for monitoring progression of CHF by monitoringinter-ventricular conduction time are provided.

[0120] While the invention has been described with reference toparticular embodiments, modifications could be made thereto by thoseskilled in the art without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A system for automatically verifying bi-chambercapture, comprising: pulse generating means for selectively generatingstimulation pulses of a predetermined energy to a patient's heart;sensing means for sensing a composite bi-chamber cardiac signalsubsequent to the delivery of the stimulation pulses, the compositecardiac signal having characteristics that represent the patient'scorresponding chambers of the heart; and control means, coupled to thepulse generating means and the sensing means, for determining whenbi-chamber capture occurs and for increasing the predetermined energy ofthe stimulation pulses when bi-chamber capture is absent.
 2. The systemaccording to claim 1 , wherein the pulse generating means comprises:means for generating stimulation pulses in each of the correspondingchambers of the patient's heart.
 3. The system according to claim 2 ,wherein the control means comprises: means for independently increasingthe predetermined energy of the stimulation pulses in each of thecorresponding chambers of the patient's heart.
 4. The system accordingto claim 1 , further comprising: means, coupled to the sensing means,for determining when bi-capture capture does not occur following astimulation pulse due to sub-threshold stimulation in at least onechamber; means, coupled to the sensing means, for determining whencapture occurs in each of the corresponding chambers.
 5. The systemaccording to claim 1 , wherein the sensing means comprises: means forsensing at least one characteristic of the cardiac signals occurring inboth chambers, the characteristic providing an indication of capture ornon-capture of two corresponding chambers of the patient's heart.
 6. Thesystem according to claim 5 , wherein the sensing means comprises meansfor sensing at least one of the following characteristics in thecomposite bi-chamber cardiac signal: a template representation of anoverall intra-cardiac electrogram (IEGM) waveform; a peak negativeamplitude; a peak positive amplitude; a positive slope; a negativeslope; a positive integral; a negative integral; a plurality ofinflection points; an average time duration of a plurality ofdepolarizations; a time interval between a stimulation pulse and anysubsequently detected event; and a time interval between consecutivelydetected events.